Methodology:

Instruments were placed at five levels on each tower (~0.3, 0.65, 1.22, 1.79 and 2.32 m above the surface, remeasured at each visit). For each, we used five NRG 40 cup anemometers, one NRG 200P wind vane and five shielded and passively-ventilated Extech RHT10 temperature and humidity loggers. Data were recorded on Campbell CR1000s with a 12 V battery stored at the base of each tower. Wind tower data were processed in MS Excel and then MATLAB.

For photogrammetric surveys, camera specifications and survey area geometry were used to calculate the footprint of each image, which determined the distance between images required to achieve 60-80% overlap, and the number of images needed for the survey area. Images were predominantly nadir and followed a regular grid pattern, with an additional ~10% of images taken obliquely (< 20° off nadir). All survey plots were marked out using a regular grid of ground control points (GCPs), the locations of which were recorded using a Leica GS10 differential GPS, with a sub-centimetre mean accuracy for each plot. Data were processed using Agisoft Photoscan Professional Edition Version 1.4.0. Dense point clouds were imported into CloudCompare 2.10 for manual cleaning, and digital elevation models were created using linear interpolation with nearest non-empty neighbours, ensuring a regular grid shape with the top of the grid aligned with the direction of flow (roughly South-North). 

A permanent in-situ Riegl VZ-6000 TLS was used to survey the majority of the glacier ablation zone using a near-infrared wavelength (1050 nm) suited to snow and ice surfaces (University of Innsbruck, 2020a). The TLS is housed in a climate-controlled container near the summit of "im Hinteren Eis" (46.79586° N, 10.78277° E, 3244 m a.s.l.). The point acquisition rate was approx. 23,000 points per second. Horizontal and vertical spatial resolution was ~0.17 m at 1000 m range, giving a theoretical density of 10 points per m2 mid-glacier, and 2 points per m2 at the head of the accumulation zone and near the terminus (University of Innsbruck, 2020b).

Regional (gridded) elevation data were acquired for the entire Austrian Alps at 10 m pixel-1 resolution (Open Data Austria, 2020). This regional product was created by interpolation of 2.5 m ALS data obtained during flights over the Ötztal Alps, Tyrol, from 2006-2012 (Fritzmann et al., 2011; Sailer et al., 2012; Fischer et al., 2015). ALTM 3100 and Gemini ALS sensors were used in the Tyrol area, with an average density of 0.25 points m-1. TLS and ALS data were processed in CloudCompare 2.10.

Data collection:

Instrumentation

NRG #40 cup anemometers

NRG 200P wind vane

Extech RHT10 temperature and humidity loggers

Campbell CR1000 data loggers

Leica GS10 differential GPS

DJI Phantom 3

Olympus OMD EM-10 camera

Riegl VZ-6000 Terrestrial Laser Scanner (TLS)

ALTM 3100 and Gemini Airborne Laser Scanner (ALS) sensors

Software

Agisoft PhotoScan Professional Edition (version 1.4.0)

CloudCompare 2.10

ArcGIS ArcMap 10.6

Matlab R2019b

Resolution

Small plots: DEMs at resolutions of 0.005, 0.01 and 0.05 m pixel-1

Large/UAV plot surveys: 0.01, 0.05, 0.1, 0.5 and 1 m pixel-1

Glacier-scale TLS/ALS: 5, 10, 20 and 30 m pixel-1

Data quality:

Wind speed is ±0.05 m s-1

Wind direction is ±1 %

Temperature is ±0.1 °C

GPS accuracy <1 cm

Photogrammetric 3D root mean square (RMS) error was ±0.03 m and RMS re-projection error was 1.66 pixels (2.6 µm)

NoData values used either NaN in Matlab or -32767 as is convention in ArcMap.